Decoking process for a pyrolysis reactor



April 2l, 1970 J. HAPPEL ET AL DECOKING PROCESS FOR A PYROLYSIS REACTOR B SheetS-Sheet l Filed Nov. 50, 1966 INVENTORS `lOl-1N RAPPEL LEONARD KRAMER G.R. KOEHLER ATTORNEY.

April 21, 1970 J. HAPPEL.v ET AL DECOKING PROCESS FOR A PYROLYSIS REACTOR 5 Sheets-Sheet 5 Filed Nov. 30. 1966 .v .UE

United States Patent O 3,507,929 DECOKING PROCESS FOR A PYROLYSIS REACTOR John Happel, 69 Tompkins Ave., Hastings-on-Hudson,

N.Y. 10706; Leonard Kramer, 2031 Central Road,

Fort Lee, NJ. 07024; and George R. Koehler, 506

Prospect Ave., Laurence Harbor, NJ. 08879 Filed Nov. 30, 1966, Ser. No. 597,925 Int. Cl. C07c 11/24; C10g 9/12; B01j 1/00, 7/00 U.S. Cl. 260-679 Claims ABSTRACT 0F THE DISCLOSURE A process for the removal of coke formed during high temperature pyrolysis processes in which hydrogen at high temperature is passed through the system, the ow of hydrocarbon feed gas having been interrupted, the surfaces of the reactor which are in Contact with coke and the reacting gases consisting of refractory carbides, borides, and mixtures thereof, and a process for pyrolysis of hydrocarbons to acetylene which employs the above defined step for removal of coke from the reactor system.

This invention generally relates to an improvement in high temperature cracking processes and more particu- Alarly relates to an improvement in pyrolysis processes which substantially removes and may eliminate coke deposits on faces in the pyrolysis and quenching zones.

In any process wherein a hydrocarbon or hydrocarbon containing mixture is heated to tempratures suiciently high such that substantial decomposition or cracking occurs, a certain portion of the combined or organic carbon present in the hydrocarbon is converted to its elemental state. In other words, it is converted to coke. The amount of this carbon which forms varies from almost none to almost 100% of the carbon in the hydrocarbon, depending upon the severity of the conditions to which the hydrocarbon is exposed. The carbon or coke which forms may contain small amounts of combined or adsorbed hydrogen, but it is a solid, which may for instance, form in the gaseous phase and leave the reactor with the exit gases as a dust. However, a certain amount of it may also plate out or form and cake on solid surfaces in the reactor. Under pyrolysis conditions, this deposit is usually hard, tenacious and difficult to remove. It is this type of carbon deposit within reactors with which this process is concerned. It is usually referred to as coke or the coking by-products of the pyrolysis reaction.

This problem of coke forming in and on the pyrolysis and quenching equipment is always a serious one, but is somewhat less serious for processes which employ temperatures in the range of about 1000" C. and below this level. At these temperatures, the coke can be removed in several lconventional ways, for instance, by substituting for the hydrocarbon feed an oxidizing gas which burns the coke away. Steam, air and carbon dioxide are typical gases which may -be so used. Under these conditions, the carbon leaves the reaction zone as carbon monoxide and/ or carbon dioxide along with other reaction products such as hydrogen when steam is used.

When this process of coke removal is used, the material or materials of construction must, at the temperatures and pressures of operation, resist carburization and hydrogen during the cracking process, and also, be resistant to oxidation during the coke burning process. In addition to chemical resistance, it is usually required that the material of construction for the reactor or packing within the reactor also have good mechanical properties, resistance to thermal shock and/ or spalling and high thermal conductivity under the conditions of operation.

ice

Up to about 1000 C. many metallic alloys are thus ecapable of being used. In other thermal processes when the heat of combustion of the coke partially supplies the energy to crack the hydrocarbon, refractories such as aluminum oxide or mullite have been employed, especially for packing. However, as the temperature level of operation during cracking and coke burn out increases, the number of construction materials which can be used is drastically reduced. At temperature levels of l400 C. to 1500" C. even the refractory oxides begin to suffer under attack by hydrogen, carbon or hydrocarbons during the pyrolysis or cracking process. In the range of 1500 C. to 2000 C. and higher, there are no readily available materials which can be economically used, have the good mechanical properties required, resist the attack of hydrogen, carbon and hydrocarbons, and also have oxidation resistance over long periods of operating time.

In operating processes for the pyrolysis of hydrocarbons and hydrocarbon-hydrogen mixtures at temperatures between l4502000C. as described for example in U.S. Patent 3,156,733 or 3,156,734, it has been found that when the methane flow was stopped, and the hydrogen continued to flow through the reactor, coke removal resulted. The extent of coke removal increased as time of exposure to hydrogen at operating temperatures increased.

It is an object of the present invention to provide a process for improved pyrolysis operations by removal of coke formed during the pyrolysis reaction.

It is a further object of the invention to remove carbon deposits from pyrolysis and quenching reactor systems by exposing the carbon contaminated surfaces to treatment with hydrogen.

It is another object of the invention to improve pyrolysis processes for high temperature cracking of hydrocarbons by removing coke from the reactor system.

Further objects of the invention will become evident from the description and details set forth hereinafter.

The invention comprises the partial or total removal of carbon deposition from reactor and quenching surfaces used in cracking hydrocarbons by contacting them with hot hydrogen. It is contemplated that the decoking improvement can be employed in hydrocarbon pyrolysis reactions in which temperatures of 1000 C. and above are used. It is particularly desirable that this decoking operation be used when carrying out pyrolysis operations in which temperatures of l400 C. andabove are lused. It is in these high temperature processes for cracking hydrocarbons that large amounts of carbon deposits are formed on the equipment surfaces.

Although it is not intended to limit this invention to any particular theory, the removal of coke by hot hydrogen may be produced by one or `more factors, For instance, chemical reaction may take place which results in reversing the coking process itself. Alternatively, the deposited carbon may be vaporized into the gas phase, giving a carbon vapor at high temperatures. Further, at the high temperature levels required for many pyrolysis reactions, one of the best and most economical materials of construction is graphite, itself a form of carbon. Since graphite can also be attacked by hot hydrogen, it can be used preferably if the graphite surfaces exposed to the cracking and decoking cycles are protected. This protection may be done by impregnating or coating at least the surfaces with materials such as refractory carbides and borides. Also, if materials other than carbides are used which react with carbon to form mixed carbides, these are satisfactory for so long as the mixed carbides resist hydrogen attack. It is understood, of course, that any protective coating must also withstand the temperature levels involved in the pyrolysis. Illustrative of materials which can be used are tantalum carbide, zirconium boride, zirconium carbide, titanium boride and carbide silicon carbide and their like.

If electrical energy is used to provide the energy requirement, the graphite reactor has a further advantage in that the reactor and heater can be integrated, if desired, into a single element.

It should tbe emphasized that, if one were to rely upon oxygen or oxidising agents to burn out the coke there is no material available at this time which could be used at these temperature levels since such materials would have to withstand oxidation, carburization and hydrogen.

With the herein described hydrogen decoking, oxidation resistance is no longer required, and so at least the carbides and borides, as examples, are available.

In the following examples, it is not intended that gases entering the systems described necessarily be cold; indeed, it may be preferable that any or all gases be preheated, especially if the reactors are heated electrically; thus, heat from the combustion of fuels could be used to provide preheat requirements, affording a saving in the total cost of energy.

Furthermore, in accord with effects on the reaction conditions, as the pressure of thesystem is increasedfrom atmospheric to atmospheres, the amount of methane or other hydrocarbons in equilibrium with carbon and hydrogen at these temperatures increases greatly. It is believed that this great increase may be one reason why the aforedescribed decoking cycle operates more eiciently where higher pressures are used. It is also noteworthy that the rate of carbon removal from the surfaces of the equipment is increased when higher pressures are employed.

The cyclic process of coking during cracking, and hydrogen burnout or decoking is illustrated by, but not limited to, the behavior of a methane pyrolysis reactor as shown in accompanying FIGURE 1. The pressure drop across the reactor, which increases with increasing coke deposition, is used as a significant measure of coke deposition and also its removal. Because the pressure drop is also significantly affected by temperature, gas density and flow rate, the procedure for measuring pressure drop was standardized, as follows. At the start and end of each cycle, whether cracking or decoking, the reactor was allowed to cool to room temperature. At room temperature, a calibrated flow of nitrogen gas was passed through the reactor, and the pressure drop was observed. This pressure drop is referred to as Calibrated Pressure Drop. After each cracking cycle, an increase in this Calibrated Pressure Drop indicated that coke had been deposited. If this Calibrated Pressure Drop decreased, e.g. after a decoking run, then this indicated that coke had been removed. Itis this Calibrated Pressure Drop which is plotted in FIGURE 1 vs. operating time. The lines connecting the points are not to be considered as an instant by instant change in pressure drop, but merely the general course of the integral change in Calibrated Pressure Drop for the entire time interval between the points. The conditions under which the reactor operated during this time interval, are shown as notes appended to the line. It is seen from FIGURE 1, that coke removal increases as the time of the hydrogen cycle is increased, and also that the rate of coke removal is increased as the flow of hydrogen is increased. The extent of coke removal is indicated by the degree of change in the Calibrated Pressure Drop, whereas the rate is a function both of the degree of change in the Calibrated Pressure Drop and the time interval during which this change occurs; i.e. the slope of the lines.

When hydrogen was passed through the reactor at temperatures below l800 C. coke removal either did not occur, or was too slow to be detected. At temperatures between 1900'o C. and 2150 C. the results shown in the curves of FIGURE 1 were obtained. This decoking process is operable at temperatures above 2150 C., then commences at about l850 C. at measurable rates. Also, as shown in FIGURE 2, the rate of carbon removal is shown to increase markedly as the reactor pressure is increased. Thus, there is a sharp increase in slope as the pressure is changed from approximately atmospheric to about p.s.i.g., when other conditions within the reactor are held constant.

Itis another feature of the invention that small amounts, 110% based on the hot hydrogen used, of oxidizing gases such as carbon dioxide and oxygen itself may be incorporated into the hydrogen decoking stream. The amount used may be varied, up to the amount which causes an unwanted level of oxidation to reactor surfaces. Further, it is also possible to operate this process at practical rates below 1850 C. by incorporating in the gas stream heterogeneous hydrogenation catalysts on the surfaces or homogeneous hydrogenation catalysts in the hydrogen gas stream Within the reactor.

Example 1 In processes for the pyrolysis of hydrocarbons such as, but not limited to, the cracking of methane to acetylene, as much as 20% or more of the hydrocarbon converted yields solid carbon, which may also contain small amounts of adsorbed or combined hydrogen. While most of this carbon leaves the reactor with the other products as soot, a small amount continuously deposits as coke within the reactor. Typical of, but not limited to, the type of high temperature process where this occurs is that described in U.S. Patent 3,156,733 wherein methane at subatmospheric pressures is pyrolyzed to acetylene at temperatures in the range of 1400 C. to 2000" C. This coke formation is also observed in lower temperature cracking processes such as the Wulff Process and in are cracking to produce acetylene.

Under conditions wherein coke is deposited within the reactor, it has been found that this coke can periodically be removed by alternating the pyrolysis cycle with la hydrogen decoking cycle. Using this method avoids the complete shut-down of the reactor, cooling, and mechanically cleaning or altogether, replacing the reactor, an expensive and prolonged operation.

A typical total cycle consists lirst of a pyrolysis cycle, curve P I. FIGURE 3, during which hydrocarbon is pyrolyzed to acetylene, at temperature of 1400 C. and above. During this cycle, coke slowly builds up in the reactor, yielding increasing pressure drop; at a predetermined level of pressure drop, this pyrolysis cycle is terminated. Then, the decoking cycle, Curve D-I, FIGURE 3, is started lwhen the ow of hydrocarbon is stopped, and the ow of hydrogen started. If not already above 1900 C., the temperature of the reactor is raised to the range 1900-2.l50 C., and then this cycle proceeds until the pressure drop returns to its original level of operation as at the start of the pyrolysis cycle. At this point, the reactor is returned to the pyrolysis cycle. Thus, by alternating these pyrolysis and decoking cycles, the pyrolysis process can be continued 'without coke deposits completely blocking the reactor, as occurs if the pyrolysis cycle alone is employed.

If the hydrocarbon cracking process employs hydrocarbon feed diluted with for example hydrogen, such as, but not limited to, a process Ias described in U.S. Patent 3,156,734, the same cyclic process is used; however, in this case, stopping the flow of hydrocarbon only, automatically converts the feed to hydrogen. Therefore, after the hydrocarbon flow is stopped, only the temperature must be adjusted to put the reactor on a decoking cycle.

Example 2 In processes wherein hydrogen is used as a diluent in the pyrolysis of hydrocarbons to acetylene, lagain as illustrated but not limited to the process described in U.S. Patent 3,156,734, the following scheme can be conveniently employed as illustrated in accompanying FIGURE 4.

In FIGURE 4 are shown two reactors, reactor #1 and reactor #2. These reactors are coupled at point #12.

During the first cycle, cycle A, hydrogen from a metered controlled hydrogen source, line 4, is admitted through valve 5, via line 6 to the hydrogen preheater 7, where its temperature is raised to 1900 C. or higher. Valve 8 is closed. The quench inlet valve 9 and product exit valve 10 are closed. Thus, hot hydrogen is admitted to first reactor 11 for a decoking cycle. On leaving reactor 11, the hydrogen is mixed at point 12, with hydrocarbon from a controlled, metered hydrocarbon source through line 13, and then passes into second reactor 14, wherein a cracking cycle occurs. The product stream leaving reactor 14 is quenched by iiuid from valve 15, and the combined product and quench stream exit through valves 16 for further cooling and/ or processing. No flow occurs through hydrogen preheater 17, because valve 8 is closed. During the cycle A, coke previously deposited in rst reactor 11, is removed, while coke may be deposited in reactor 14. At a predetermined point, as measured by the pressure drop across reactor 14, it may be desired to remove coke deposited within this reactor 14 during cycle A.

Therefore, to convert to cycle B, valves 5, and 16 are closed; valves 8, 9 and 10 are opened. Thus hydrogen is admitted to preheater 17 via line 18, and then to reactor 14 for decoking. Hot hydrogen leaving reactor 14, is then mixed at point 12 lwith hydrocarbon from line 13, and the mixture fed to reactor 11, which was decoked during cycle A, and which is now on a cracking cycle. The exit product from reactor 11 is quenched by fluid from valve 9, and the combined product and quench streams leave through valve 10 to further cooling or processing. No gas flows through preheater 7, because valve 5 is closed.

Energy is supplied to all preheaters and reactors in accordance with each of their loads, such that, they are maintained at a predetermined temperature as follows:

Preheaters-over 1900 C. Reactors-during cracking, in accord with process employed; during decoking, over 190 C.

If the decoking cycle be shorter than or equal to the cracking cycle, this arrangement permits essentially all the heat added to the hydrogen for the decoking, to be utilized at high temperature levels for feeds preheating.

If, however, the decoking cycle is longer than the cracking cycle, then a third cycle, cycle C, is introduced into the total cycle, during which hydrogen is passed through both reactors.

Thus, valve on line 13 can be closed during cycle C, and the temperature of reactor 11 raised to over 1900 C., if it is not already at that level. Hydrogen is now permitted to ow through both reactors and both are on a decoking cycle. Alternatively, valves 9, 10 and on line 13 can be closed and valve 5 opened. Flow from the hydrogen line 4, is there adjusted to lprovide both reactors 11 and 14 with adequate ow and hot hydrogen is now exited through line 19. Where the decoking cycle is longer than the cracking cycle only part of the heat needed for hydrgen heating is recoverable.

However, in both above Examples 1 and 2, when the ow of hydrocarbon feed has been stopped and hydrogen is essentially the only product leaving the system, this hot hydrogen can be diverted to a heat accumulator such as a pebble bed or checkerwork. The heat thus stored can be used for preheating incoming streams making further economical recovery of heat possible.

Example 3 It is also possible to carry out these alternating cycles of pyrolysis and decoking using the schematic equipment shown in FIGURE 3, but with the addition of a pressure reducing device such as -a valve or orifice at point 12. Thus, methane or hydrocarbon feed with or without admixed diluent gases can be pyrolyzed at one atmosphere (abs.) or less; subsequently the pressure is raised to p.s.i.g. during the decoking cycle.

The effect, here, is to shorten the decoking cycle, thus reducing hydrogen and energy requirements. Also, as shown in FIGURE 3, curve II, the total cycle time is reduced, thus increasing the productivity of a given sized reactor per unit time of operation, as compared with curve I.

What is claimed is:

1. A process for the pyrolysis of hydrocarbons to acetylene in a graphite reactor wherein at least the surfaces in contact with reacting gases are protected by impregnating or coating with material selected `from the group of refractory carbides, borides and mixtures thereof, to prevent reaction of the graphite with hydrogen contacted therewith during a coke removal cycle.

2. A process according to claim 1 for the pyrolysis of methane to acetylene in a graphite reactor whose surfaces in contact with reacting gases are coated with a material selected from the group of refractory carbides, borides, and mixtures thereof, to prevent reaction of the graphite with hydrogen contacted therewith during a coke removal cycle.

3. A process according to claim 1 in which the coating consists of refractory yborides.

4. A process according to claim 1 in which the coating consists of refractory carbides,

5. A process according to claim 1 in which the coating consists of a mixture of refractory borides and refractory carbides,

References Cited UNITED STATES PATENTS 1,789,131 1/1931 Benner et al. 106-44 2,354,163 7/ 1944 Weizmann et al a- 196-47 2,547,221 4/1951 Layng 252-411 3,156,734 11/1964 Happel et al. 260-679 OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 5, 1924, p. 821.

DELBERT E. GANTZ, Primary Examiner I. M. NELSON, Assistant Examiner U.S. Cl. X.R. 

